1. Field of the Invention
The invention relates to a memory, and more particularly to a phase-change memory and method for fabricating the same.
2. Description of the Related Art
Phase-change memory technology requires high reliability, fast speeds, low current, and low operating voltage, in order to function as a viable alternative to current memory technologies such as flash and DRAM. A phase-change memory cell must therefore provide low programming current, low operating voltage, a smaller cell size, a fast phase transformation speed, and a low cost. These requirements are difficult to meet given the current state of the art.
Current phase-change memory technology makes use of heating at the interface between a metal electrode contact and the phase-change material. More effective heating requires a smaller contact area, or equivalently a smaller heating area. A benefit of this strategy is simultaneous reduction of cell size. However, reducing the current-carrying area results in higher cell resistance, which increases the required driving voltage. This is clearly not desirable. Reducing heating area does not necessarily improve other performance features. There is a large temperature gradient that exists between the contact and the bulk of the phase-change material. Phase transformation speed requires good thermal uniformity within the active region of the cell. The rate of phase-change is extremely sensitive to temperature. Non-uniform heating results in a loss of reliability due to accumulation of incomplete phase-change in the programming volume.
United States Patent 20070012905 utilizes a single edge contact to the lower electrode, while the upper electrode uses a conventional planar contact. In addition, U.S. Pat. No. 6,881,603 also minimizes only the lower electrode contact area while the upper electrode contact is planar. Meanwhile, U.S. Pat. No. 6,864,503 makes use of a phase-change material spacer with top and bottom edge contacts, however, the heating area is proportional to the electrode radius, so it is relatively large, and the upper and lower electrodes are effective heat sinks.
Therefore, it is desirable to devise a phase-change memory cell structure that improves thermal uniformity as well as heating efficiency while allowing for a smaller heating area.